The present invention relates to a method and catalyst for the simultaneous removal of carbon monoxide and nitrogen oxides (NOx) contained in flue or exhaust gas.
More particularly, the invention provides a method, where flue gas or exhaust gas containing harmful carbon monoxide, organic compounds (VOC) and NOx is contacted with a layered catalyst in which a first layer comprises an oxidation catalyst and in an underlying layer a NH3-SCR catalyst for the simultaneous removal of the carbon monoxide and NOx.
Removal of NOx, VOC and CO from flue or exhaust gas is conventionally exercised by use of two different catalyst compositions, wherein an oxidation catalyst is arranged upstream of an SCR catalyst with injection of a reductant between the catalysts. NOx removal is typically performed by selective catalytic reduction (SCR) with NH3 on vanadium oxide or zeolite-based catalysts in monolithic form. Ammonia is injected upstream the SCR catalyst and reacts with the NOx on the catalyst surface. An optimal temperature window for the vanadium oxide-based catalysts is 200-400° C., while zeolite based catalysts are more active at temperatures >400° C.
In the case of CO and VOC removal by catalytic oxidation, the platinum metals are the most common choice due to their high reactivity already at temperatures >200° C.
As an example of a flue gas containing both CO, VOCs and NOx is the flue gas from a turbine operating on natural gas. Traditionally, in the HRSG design the CO oxidation catalyst, often based on Pt, is located upstream the SCR catalyst and the ammonia injection grid (“AIG”). This location has been chosen mainly due to the fact that the oxidation catalyst is very active in the oxidation of NH3 to NOx, which is highly undesired. Having the CO oxidation catalyst located upstream the AIG makes sure that no NH3 is wasted, but all amounts of injected ammonia reach the SCR catalyst limiting the operation costs of the utility.
In an alternative configuration, the oxidation catalyst is arranged downstream the SCR catalyst. When positioned this way the oxidation catalyst is operated at lower temperatures than the conventional layout. The problem with this configuration is that if not designed correctly, the oxidation catalyst may oxidize the NH3 slip to NOx, thus reducing the overall NOx removal of the plant. Possibly, the oxidation catalyst may be designed in a way that NH3 is converted to N2 instead, but such a catalyst is typically more expensive than a conventional oxidation catalyst due to both the kind and quantity of the noble metals used for its production.
In the above configurations, the resulting reactor consists of two separate catalyst units, i.e. one SCR catalyst unit and one oxidation catalyst unit. More precisely, the total volume of catalyst installed will be determined by the size of the SCR catalyst unit, plus the size of the oxidation catalyst unit.
In order to reduce the size of the reactor, a combination of the two catalysts partly on the same support has been attempted and in some cases accomplished.
U.S. Pat. No. 7,390,471 discloses an exhaust gas treatment apparatus for reducing the concentration of NON, HC and CO in an exhaust gas stream. The treatment apparatus includes a multifunction catalytic element having an upstream reducing-only portion and a downstream reducing-plus-oxidizing portion that is located downstream of an ammonia injection apparatus. The selective catalytic reduction (SCR) of NOx is promoted in the upstream portion of the catalytic element by the injection of ammonia in excess of the stoichiometric concentration with the resulting ammonia slip being oxidized in the downstream portion of the catalytic element. Any additional NOx generated by the oxidation of the ammonia is further reduced in the downstream portion before being passed to the atmosphere. The reduction-only catalyst may be vanadium/TiO 2 and the reduction-plus-oxidizing catalyst includes a reduction catalyst having 1.7 wt percent of vanadium/TiO2 impregnated with 2.8 g/ft 3 each of platinum and palladium.
However, the SCR activity of the oxidation catalyst is considerably lower than the SCR activity of an SCR-only catalyst meaning that the total volume of catalyst installed will be equal to the volume of the oxidation catalyst plus the volume of the SCR catalyst needed to compensate for the low SCR activity of the oxidation catalyst.
In the cleaning of gas turbine flue gas as an example, number one priority from a utility point of view is to reduce the total catalyst volume as much as possible. Large volumes in fact mean high pressure drop across the catalyst bed and overall lower efficiency of the HRSG. The pressure drop has a direct impact on the net power achievable from the turbine and an in-direct effect on the heat flux, i.e. the calories that can be extracted from the flue gas by the HRSG.
In order to reduce the catalyst volume to a minimum, the SCR activity of the oxidation catalyst has to be increased to the same high levels of an SCR-only catalyst. One essential condition for obtaining this is the use of an oxidation catalyst very active in the oxidation of CO and VOC, but not reacting with NH3. Another important condition is that the oxidation catalyst must still have the same oxidation activity as an oxidation-only catalyst.